LAS VEGAS — Donald J. Trump has shaken up his presidential campaign for the second time in two months, hiring a top executive from the conservative website Breitbart News and promoting a senior adviser in an effort to right his faltering campaign.

Stephen Bannon, the executive chairman of Breitbart News LLC, will become the Republican campaign’s chief executive, and Kellyanne Conway, a senior adviser and pollster for Mr. Trump and his running mate, Gov. Mike Pence of Indiana, will become the campaign manager.

Paul Manafort, the campaign chairman, will retain his title. But the staffing change, hammered out on Sunday and set to be formally announced Wednesday morning, was seen by some as a demotion for Mr. Manafort.

People briefed on the move said that it reflected Mr. Trump’s realization that his campaign was at a crisis point. But it indicates that the candidate — who has chafed at making the types of changes his current aides have asked for, even though he had acknowledged they would need to occur — has decided to embrace his aggressive style for the duration of the race.

Both Ms. Conway and Mr. Bannon, whose news organization has been very favorable to Mr. Trump since he entered the primaries, are close with Robert and Rebekah Mercer, the father-and-daughter conservative donors who have become allies of the candidate and are funding a “super PAC” that is working against Hillary Clinton.

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Mr. Bannon has no experience with political campaigns, but he represents the type of bare-knuckled fighter that the candidate had in Corey Lewandowski, his combative former campaign manager, who was fired on June 20.

Mr. Bannon has been a supporter of Mr. Trump’s pugilistic instincts, which the candidate has made clear in interviews he is uncertain about suppressing. He is also deeply mistrustful of the political establishment, and his website has often been critical of Speaker Paul D. Ryan and Senator Mitch McConnell, the majority leader.